The present invention relates to a rotation angle detection device equipped with a rotating main gear, for detecting a rotation angle of the main gear.
As a prior art, there is a known technology that allows a steering angle of a steering wheel to be detected uniquely even if the steering angle of the steering wheel exceeds 360 degrees (see Japanese Patent Application Laid-open No. 2002-531858). In detail, the above technology enables the steering angles of 370 degrees and 10 degrees to be distinguished from each other.
In this technology, a rotation angle detection device disclosed therein includes a main gear, first and second detection gears, first and second detection magnets, first and second magnetic sensors and a microcomputer.
In the rotation angle detection device, the main gear rotates integrally with a steering shaft of a vehicle, while the first and second detection gears rotate in conjunction with the main gear.
The first detection magnet, which is a magnet magnetized with two poles, rotates together with the first detection gear. Similarly, the second detection magnet also magnetized with two poles rotates together with the second detection gear.
The first magnetic sensor detects the directions of magnetic lines of the first magnet fixed on the first detection gear, that is, the rotation angle of the first detection gear and further outputs a detection signal in accordance with the detection result (i.e. detection angle). Similarly, the second magnetic sensor detects the directions of magnetic lines of the second magnet fixed on the second detection gear, that is, the rotation angle of the second detection gear and further outputs a detection signal in accordance with the detection result (i.e. detection angle).
The microcomputer calculates the rotation angle of the main gear based on the detection signals outputted from the first and second magnetic sensors.
Here noted that the number of teeth (or “teeth number”) m1 of the first detection gear and the number of teeth (teeth number) m2 of the second detection gear are respectively established so as to meet a condition 1 or a condition 2 as follows:m2=m1+1  (condition 1)|m1−m2|>1 (m1 and m2 are prime to each other)  (condition 2)
When the numbers of teeth m1, m2 meet the above condition 1 or the condition 2, the combinations of detection signals from the first magnetic sensor with detection signals from the second magnetic sensor differ from each other so long as the rotation angles of the main gear corresponding to the combinations differ from each other, within the rotating range of the main gear, in other words, the steering range of a steering wheel. Therefore, if only given the detection signals from the first and second magnetic sensors, then it becomes possible to determine the rotation angle of the main gear uniquely.
That is, according to the above technology, even if the steering angle of the steering wheel exceeds 360 degrees in such a system that detects the rotation angle of the main gear on the ground of the rotation angles detected by the first magnetic sensor and the second magnetic sensor, it is possible to detect the rotating angle of the main gear, namely, the steering angle of the steering wheel uniquely.
However, the following problem resides in the above-mentioned technology. That is, as the detection gears used in the technology are respectively limited to ones meeting the above condition 1 or the above condition 2, the manufacturing of a device according to the technology requires the selection of gears that can be accommodated in such a device and further meet the condition 1 or the condition 2.
Thus, if there is a limitation in size of the device, it is complex to select appropriate gears.